spi: rockchip: add power-domains property

The Rockchip rk3399 TRM mentions that pd_sdioaudio includes
sdio, spi, i2s and spdif. Add a power-domains property to
to match reality with spi-rockchip.yaml.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Acked-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/12af4fe6-4d35-cb4a-f5f6-06e3aba990cb@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>